Final answer:
The secretion from sweat glands contains lactic acid and fatty acids. Sebaceous glands produce sebum, but it does not contain lactic acid. The correct answer to the student's question is sweat gland secretions.
Step-by-step explanation:
The skin secretion that contains lactic acid and fatty acids is the sweat gland secretions. The sebaceous glands produce an oily substance called sebum, which lubricates and waterproofs the skin and hair. However, it does not contain lactic acid. Instead, sweat glands, particularly eccrine sweat glands, secrete a salty fluid called sweat that contains lactic acid, which can be found throughout the skin, except for where hair does not grow such as on the palms and soles.